Sometimes people have a small job for a computer. To complete this small job, they write some simple directions for the computer. But if their directions are good, or other people want to do the same job, other people will want to use their directions too.
But if they didn't plan for lots of people to use their directions for the computer, they might be in trouble. The directions they wrote might be slow, or they might not work right for other people. They might have to make a whole new set of directions that works for everyone.
My job is to make it so you can write one set of directions, and keep using it the whole time. For this to work, you have to be able to change the directions to work better for everyone, or to run faster, or to use lots of computers at once. But changing the directions shouldn't mean that you need a whole new set of directions. Instead, you should just change part of the directions at a time. And even when you have changed part of the directions, but not all the other parts, all the parts of the directions should work together and the part you changed should work better, just like you wanted.